Electrophotographic image forming apparatuses are widely used. It is because a clear image can be obtained in a short time in comparison with image forming apparatuses using other systems such as an inkjet system.
Adopted as the image forming method of the electrophotographic image forming apparatuses is an intermediate transfer system, and a belt unit (transfer unit) provided with a belt is used in the intermediate transfer system. In the image forming process of the intermediate transfer system, toner adhering to a latent image is temporarily transferred to the belt in the transfer unit, and afterwards transferred from the belt to a medium such as paper.
Concerning the configuration of an image forming apparatus that adopted the intermediate transfer system, various proposals have been already made. Specifically, in order to detect the amount of displacement etc. of the belt, a detection mark (belt marking member) is made on the belt (e.g., see Patent Document 1). This detection mark is detected by an optical reflection sensor (mark detecting sensor).